Low Concrete Testing

[SteelPE]

This is an expansion on the thread I had a few weeks ago regarding low testing reports I received on a recent project.

Just today, on a completely different project, for a different client, with a different contractor and a different concrete plant, I received a 7 day break on some footings that is trending low (about 60% of the design compressive strength). I am skeptical that this set of test will achieve the design compressive strength. While I am probably jaded from my previous experience with low concrete testing, and I would like to be wrong, I am wondering what to do about this impending issue? This is like watching a car accident about to happen in slow motion. We won't know the official results of the concrete for another 21 days, but in that time this area will have walls and piers on them.

I have notified the GC to let them know that we may have an issue in the future. They have contacted the testing agency who "believes the concrete will attain the necessary strength at 28 days". This is akin to Schrodinger's Concrete..... until we know for sure, we don't know.

 

[Enable]

I might consider restricting the rate at which they can build considering the reduced strength of the footing (if you think that the strength is really required for the loads and not simply selected from a table as a correlate of durability) and issue a note to the GC saying that it is a potential structural issue; the note should also include that the GC is proceeding at their own / sole risk. However, in general, I would wait for the 28-day to do anything drastic (e.g. issue a stop work order). In my experience, when you speak with the ready mix supplier their response is always "just wait longer". To be fair to them, there could have been a problem with the break, and this is why we take spares. But knowing construction they probably only have one of those and you'll want to save it for a 56 day break in the event of an issue.

If it's still an issue at 28-days, you can attempt to use in-situ cores to pass muster. In my experience, most cored samples pass this requirement (also use that spare for 56 days as above).

 

[jhnblgr]

What was the air?
How were the cylinders cured?
Take cores for testing

 

[EZBuilding]

I might consider restricting the rate at which they can build considering the reduced strength of the footing

However, in general, I would wait for the 28-day to do anything drastic (e.g. issue a stop work order).

I would not take this approach. It has been drilled into me that as the Structural Engineer of Record we do not control or provide such direction to the contractor. You could be opening yourself up to delay claims by the contractor, should you chose to restrict the rate of construction or issue a work order.

I would assume that your contract is with the Architect or Owner, to whom you may provide notice of the possible future issue. I would probably not take any action until I have been notified that the concrete strength does not meet specifications.

 

[Enable]

I would not take this approach. It has been drilled into me that as the Structural Engineer of Record we do not control or provide such direction to the contractor. You could be opening yourself up to delay claims by the contractor, should you chose to restrict the rate of construction or issue a work order.

I would assume that your contract is with the Architect or Owner, to whom you may provide notice of the possible future issue. I would probably not take any action until I have been notified that the concrete strength does not meet specifications.

If you believe their is a risk to life safety (e.g. construction failure due to low strength) you absolutely have a professional obligation to do such things. I would not think a footing would tend to lead to such an issue, but we are not aware of the particulars, and it very well might.

As I mentioned, in general the most common approach is to bring it up as a potential issue and let the contractor know they proceeding at own risk (unless quite certain life safety issue, then see above). Perform further tests as required thereafter.

 

[JAE]

Most 7 day breaks land in the 60% to 75% of specified f'c. This wouldn't concern me at all to be honest. See what the 28 day breaks reveal and then act according to the ACI or CSA codes.

Usually specifications or notes state that loading concrete before it reaches 75% of f'c is not allowed. You might remind them of this depending on if the concrete member is expected to actively support loads from the 7 day point onward.

 

[Michel60]

At 60% @ 7days it's still likely to be close to design strength @ 28days. In the interim taking more frequent and/or additional cylinders during placements will help establish if the test is an outlier or an indication of a mix needing adjustments.